Method for Preparing Optical Waveguide Substrate This invention relates to a method for preparing an optical waveguide substrate having transparent quartz films on the surface and in the interior, exhibiting improved electrical insulation and optical propagation, and suited for use in optical waveguide devices for optical communication.
Quartz substrates and silicon substrates are common substrates for use in optical waveguide devices for optical communication. The silicon substrates are typically used in the manufacture of semiconductor integrated circuits and characterized by a good heat conductivity and surface amenability to a variety of processes including etching, oxidation and deposition. They are available in large size and at a low cost.
In one prior art known method for preparing a waveguide device, a glass film, known as under clad film, is formed on a silicon substrate to a thickness of about 10 to 30 xcexcm by a flame deposition technique. In addition to the deposition of a glass film on a silicon substrate, there is also known a thermal oxidation method of directly oxidizing a silicon substrate to form a glass film.
Referring to FIG. 2, the process of preparing an optical waveguide circuit by the flame deposition technique is described. An under clad layer 2 is first formed on a substrate 1, and a core film 4 is formed thereon. The core film 4 is processed to define a waveguide pattern of protrusions 5, which is designated a core pattern. Thereafter, an upper clad glass film 6 is formed thereon. This process produces a buried optical waveguide. This process is herein designated protrusion process.
Another process of preparing a similar buried waveguide pattern is disclosed in JP-A 10-197737. Referring to FIG. 3, an under clad layer 2 is formed on a substrate 2, then processed in accordance with a waveguide pattern to define a recess 3. A core film 4 is formed thereon. The core layer 4 is then abraded off until the upper surface of the clad layer 2 is exposed, leaving a core segment 5 as shown in FIG. 3(e). A glass film 6 serving as an upper clad layer is then formed to produce a buried optical waveguide. This process is herein designated depression process.
The depression process has advantages over the protrusion process. Since the optical circuit pattern is formed as recesses by etching, the depression process eliminates thinning of the core pattern during etching which can occur in the protrusion process. Since the core is buried in the under clad during formation of the upper clad, the upper clad applies less stresses to the core than in the protrusion process.
For the formation of the under clad in the depression process, a flame deposition technique is used to deposit a glass film of about 30 xcexcm thick. Due to differential thermal expansion between glass and the substrate, a warpage of about 200 xcexcm can occur on the glass side when the substrate has a diameter of 4 inches. Thereafter, the glass film in warped state is etched to define recesses in accordance with the core pattern. A core film of about 8 xcexcm thick is deposited and vitrified on the etched surface. The core film is abraded from its upper surface until the core film is separated into segments buried in the recesses. In this process, the formation of the core pattern on the substrate in warped state is followed by the abrasion of the core film. Unless a suitable countermeasure is taken for correcting the warp, uniform etching and abrasion of the entire wafer is impossible. One common approach is to vacuum chuck the wafer on the back side, but fails to correct the entire wafer for complete uniformity. The vacuum chuck requires to-abrade wafers one by one, which is industrially inadequate.
An object of the invention is to provide a simple method for effectively preparing an optical waveguide substrate featuring a low loss.
The invention provides a method for preparing an optical waveguide substrate, comprising the steps of:
forming concave grooves in one surface of a silicon substrate in accordance with the pattern of a desired waveguide device,
thermally oxidizing the silicon substrate to form a peripheral quartz layer surrounding the grooves,
burying in the grooves a doped quartz glass layer having a higher refractive index than the peripheral quartz layer,
abrading the surface of the resulting structure to be flat to define higher refractive index quartz glass layer segments in the grooves, and
forming on the flat surface a glass layer having a lower refractive index than the quartz glass layer, wherein light propagates through the higher refractive index quartz glass layer segments buried in the grooves.
Using thermal oxidation in combination with a recessed substrate, the invention has succeeded in fabricating an optical waveguide substrate in which the under clad layer and the core layer are alternately arranged on the same surface of the substrate. The core pattern is not deformed since the core is flanked with the thermally oxidized layer. Also the thermally oxidized layer is a pure quartz layer which minimizes an insertion loss.
The substrate having alternate under clad and core undergoes minimized warp because the under clad layer is formed by thermal oxidation. This facilitates the abrasion step.